The internal experience of upper register singing, in cricothyroid dominant singing voice, or in the middle register for sopranos and mezzos, sometimes referred to as 'mixing head and chest voice' or 'mix voice' (though I feel that those phrases are misleading) can often sound very different to us internally.
This is a problem because if we try to make the internal sound sound like what we think it should sound like, we can easily end up pushing and squeezing the notes out in an effort to make it sound brighter and more powerful.
So what is one to do? If you have a teacher, talking about and working on the registration event is a great way to spend a lesson! If you don't have a teacher, this is one of the reasons I suggest recording oneself semi-regularly.
During your practice, focus on making a smooth transition over the registration event. In the beginning, it may sound especially hollow in your head. If you have belted, intentionally or otherwise, in the past, it is likely to feel very weak. Record yourself, just a scale or two, and have a listen. It may be that the registration event is still prominent, it will take some practice to smooth it over.
The one thing that you must not hear is an 'airy' voice. That is an indication of vocal fold separation, and will not result in a smooth connection across the registration event regardless of how much you practice it. You are practicing configuring the vocal folds in a fundamentally different way; one that will not produce a seamless registration event as the vocal folds are allowed to abduct for that airy sound. A smooth registration event requires vocal fold adduction, though adjusted to allow for cricothyroid dominance.
